1. Field of the Invention
The present invention generally relates to an automotive power plant and, more particularly, to a fuel control system for an automobile internal combustion engine designed to reducing quickly and properly the amount of fuel to be injected into the combustion chamber during an deceleration of the combustion engine.
2. Description of the Prior Art
With the progress of an electronic technology, a majority of recent models of automotive power plants employ a fuel injection system in place of a carburetor system. As is well known to those skilled in the art, the fuel injection system includes a fuel injector operable under a control of a control unit such as a microcomputer to inject a controlled amount of fuel into the combustion chamber. The control unit performs a calculation based on the amount of air flowing through an air intake passage leading to the engine, so that an air-fuel mixture of an air-fuel mixing ratio appropriate to the particular engine operating condition can be eventually supplied into the combustion chamber to achieve a proper combustion of the air-fuel mixture.
In this well known system, the use has been made of an air flow sensor disposed on the air intake passage intermediate between an air suction opening or an air cleaner and a throttle valve for detecting, and providing the control unit with an air signal indicative of, the amount of air actually flowing through the air intake passage. So far as the air signal outputted from the air flow sensor is utilized to permit the control unit to calculate the amount of fuel to be injected and then to drive the fuel injector, it has often been observed that a deviation occurs between the fuel injection timing, at which the controlled amount of fuel is actually injected, and the timing at which the air flowing through the air intake passage actually reaches the combustion chamber, due to, for example, a delay in detecting the amount of air flowing in the air intake passage and/or a delay of the control unit to complete the required calculation.
The deviation referred to above is substantially considerable particularly during the acceleration and deceleration of the combustion engine, and it is not infrequent that the amount of fuel injected during the acceleration becomes short of the requirement and that, during the deceleration, the amount of fuel injected becomes excessive relative to the amount of air supplied into the combustion chamber, that is, the air-fuel mixture becomes enriched. The supply of an insufficient fuel into the combustion chamber such as occurring during the acceleration of the engine may lead to a failure of the air-fuel mixture to burn properly and, on the other hand, the supply of the enriched air-fuel mixture such as occurring during the deceleration may result in not only a failure of the air-fuel mixture to burn properly, but also an after-burning in an exhaust system.
To avoid the problems discussed above, it has been suggested to employ the throttle opening, i.e., the opening of a throttle valve adjustable within the air intake passage between a full open position and a substantially closed position, as a parameter by which any one of the amount of air to be supplied into the combustion chamber and the amount of fuel to be injected into the combustion chamber can be corrected or rectified to eventually provide a properly adjusted air-fuel mixture. By way of example, the Japanese Laid-open Patent Publication No. 62-223432 published Oct. 1, 1987, deals with the problems which would occur during the deceleration of the combustion engine. According to this publication, it is suggested to determine, in reference to the throttle opening, a load imposed on the engine shortly before the deceleration takes place and then to increase the amount of air supplied into the combustion chamber thereby to avoid an enrichment of the air-fuel mixture.
On the other hand, the Japanese Laid-open Patent Publication No. 62-206246 published Sep. 10, 1987, deals with the problems which would occur during the acceleration of the combustion engine. According to this publication, it is suggested to estimate, based on the throttle opening, the amount of air which would be supplied into the combustion chamber, so that the amount of fuel to be injected into the combustion chamber during the acceleration can be increased in dependence on the difference between the estimated amount of air and the amount of air actually detected by the air-flow sensor.
In any event, the conventional fuel control system for the internal combustion engine is so designed and so structured that the amount by which the fuel to be injected into the combustion chamber is adjusted (which amount is hereinafter referred to as an amount of correction of the fuel) to eventually provide the air-fuel mixture of a proper mixing ratio appropriate to a particular engine operating condition can be calculated in reference to an amount of change of the throttle opening during the deceleration and, therefore, a delay in fuel injection timing tends to occur to an extent corresponding to the time during which the calculation takes place. This delay tends to result in a failure of the air-fuel mixture to burn properly or an occurrence of the after-burning in the exhaust system.
If the amount by which the fuel is adjusted, i.e., the amount of correction of the fuel, in an attempt to provide the air-fuel mixture of a proper mixing ratio appropriate to the decelerating condition of the engine is fixed at a constant value, the above discussed problem may be substantially eliminated. However, this contemplated method would not result in the mixing of fuel with air in a proportion appropriate to the deceleration of the engine with the combustibility thereof deteriorated consequently.
In view of the foregoing, in order to minimize or substantially eliminate any possible occurrence of the after-burning in the exhaust passage or the failure of the air-fuel mixture to burn within the combustion chamber during the deceleration of the engine, the amount of fuel being injected has to be reduced as quickly as possible at an early stage of a return of the throttle valve towards the closed position within the air intake passage, or the amount of correction of the fuel will not follow up a change in amount of air being sucked towards the combustion chamber.
On the other hand, in order to achieve an accurate reduction in amount of the fuel being injected it is necessary to reduce the amount of fuel in proportion to a change in load on the engine. However, if the amount of fuel is reduced in proportion to the change in load on the engine, a similar problem would occur, that is, the amount of correction of the fuel will not quickly follow up a change in amount of air being sucked towards the combustion chamber.